The present invention relates to improved suspended particle device (xe2x80x9cSPDxe2x80x9d) films, and to light valves comprising such films.
Light valves have been known for over sixty years for the modulation of light. As used herein, the term xe2x80x9clight valvexe2x80x9d is defined as a cell formed of two walls that are spaced apart by a small distance, with at least one wall being transparent. The walls are provided with electrodes thereon, usually in the form of transparent, electrically conductive coatings. The cell contains a light-modulating element (sometimes herein referred to as an xe2x80x9cactivatable materialxe2x80x9d), which may be either a liquid suspension of particles or a plastic film in which droplets of a liquid suspension of particles are distributed.
The liquid suspension (sometimes herein referred to as xe2x80x9ca liquid light valve suspensionxe2x80x9d) comprises small particles suspended in a liquid suspending medium. In the absence of an applied electrical field, the particles in the liquid suspension assume random positions due to Brownian movement, and hence a beam of light passing into the cell is reflected, transmitted or absorbed, depending upon the cell structure, the nature and concentration of the particles and the energy content of the light. The light valve is thus relatively dark in the OFF state. However, when an electric field is applied through the liquid light valve suspension in the light valve, the particles become aligned and for many suspensions most of the light can pass through the cell. The light valve is thus relatively transparent in the ON state. Light valves of the type described herein are also known as xe2x80x9csuspended particle devicesxe2x80x9d or xe2x80x9cSPDsxe2x80x9d.
Light valves have been proposed for use in numerous applications including, e.g., alphanumeric displays and television displays; filters for lamps, cameras, optical fibers and for displays; and windows, sunroofs, sunvisors, eyeglasses, goggles and mirrors and the like, to control the amount of light passing therethrough or reflected therefrom as the case may be. Examples of windows, without limitation, include architectural windows for commercial buildings, greenhouses and residences, windows for automotive vehicles, boats, trains, planes and spacecraft, windows for doors including peepholes, and windows for appliances such as ovens and refrigerators, including compartments thereof.
For many applications, it is preferable for the activatable material, i.e,. the light modulating element, to be a plastic film rather than a liquid suspension. For example, in a light valve used as a variable light transmission window, a plastic film, in which droplets of liquid suspension are distributed, is preferable to a liquid suspension alone because hydrostatic pressure effects, e.g., bulging, associated with a high column of liquid suspension can be avoided through use of a film, and the risk of possible leakage can also be avoided. Another advantage of using a plastic film is that in a plastic film, the particles are generally present only within very small droplets and, hence, do not noticeably agglomerate when the film is repeatedly activated with a voltage.
A xe2x80x9clight valve filmxe2x80x9d as that term is used herein, refers to a film having droplets of a liquid suspension of particles distributed in the film or in part of the film.
Light valve films made by cross-linking emulsions are known. See, for example, U.S. Pat. Nos. 5,463,491, 5,463,492 and 5,728,251, and 6,114,405, all of which are assigned to the assignee of the present invention. All of the above patents and any other patents or patent applications and references cited herein are incorporated into this application by reference thereto.
The following is a brief, non-limiting description of liquid light valve suspensions as known in the prior art, which suspensions may be modified in accordance with the present invention as taught herein to obtain improved SPD films and light valves comprising the same.
1. Liquid Suspending Media and Stabilizers.
The liquid light valve suspension may be any liquid light valve suspension known in the art and may be formulated according to techniques known to one skilled in the art. The term xe2x80x9cliquid light valve suspensionxe2x80x9d as used herein means a xe2x80x9cliquid suspending mediumxe2x80x9d in which a plurality of small particles are dispersed. The xe2x80x9cliquid suspending mediumxe2x80x9d comprises one or more non-aqueous, electrically resistive liquids in which there is preferably dissolved at least one type of polymeric stabilizer, which acts to reduce the tendency of the particles to agglomerate and to keep them dispersed and in suspension.
The liquid light valve suspension useful in the present invention may include any of the liquid suspending media previously proposed for use in light valves for suspending the particles. Liquid suspending media known in the art which are useful herein, include, but are not limited to the liquid suspending media disclosed in U.S. Pat. Nos. 4,247,175 and 4,407,565. In general, one or both of the liquid suspending medium or the polymeric stabilizer dissolved therein is chosen so as to maintain the suspended particles in gravitational equilibrium.
The electrically resistive liquids chosen for use in prior art suspending media in SPD films are typically either xe2x80x9chighxe2x80x9d molecular weight polymeric liquids or mixtures of xe2x80x9chighxe2x80x9d and xe2x80x9cmediumxe2x80x9d molecular weight polymeric liquids, as those terms (i.e., high and medium molecular weight) are defined herein, in order to obtain suspensions having optimal viscosity for a variety of coating applications. The prior art, in fact, teaches that by increasing the viscosity of the liquid suspending medium, e.g., with the use of high molecular weight (or mixtures of high and medium molecular weight) suspending liquids, the stability of the emulsion used in forming the light valve film is improved, with a resultant improvement in the properties and performance of the film produced therefrom.
The polymeric stabilizer, when employed, can be a single type of solid polymer that bonds to the surface of the particles, but which also dissolves in the non-aqueous liquid or liquids of the liquid suspending medium. Alternatively, two or more solid polymeric stabilizers may serve as a polymeric stabilizer system. For example, the particles may be coated with a first type of solid polymeric stabilizer such as nitrocellulose which, in effect, provides a plain surface coating for the particles, after which the coated particles are recoated with one or more additional types of solid polymeric stabilizer that bond to or associate with the first solid polymeric stabilizer and which also dissolve in the liquid suspending medium to provide dispersion and steric protection for the particles. Liquid polymeric stabilizers may also be used to advantage, especially in SPD light valve films, as described in U.S. Pat. No. 5,463,492.
2. Particles.
Inorganic and organic particles may be used in a light valve suspension, and such particles may be light-absorbing or light-reflecting in all or part of the visible portion of the electromagnetic spectrum.
Conventional SPD light valves have generally employed polyhalide particles of colloidal size. As used herein, the term xe2x80x9ccolloidalxe2x80x9d, when referring to particle size, means that a particle has a largest dimension averaging 1 micron or less. Preferably, polyhalide or other types of particles used or intended for use in an SPD light valve suspension will have a largest dimension which is less than one-half of the wavelength of blue light, i.e., 2000 Angstroms or less, to keep light scatter extremely low.
3. Deficiencies of Prior Art Films
Prior art cross-linkable SPD films, incorporating suspensions of the type described above, suffer from a variety of deficiencies as described herein which have prevented such films from being mass produced and from thus reaching their full commercial potential.
For example, U.S. Pat. Nos. 5,463,491 and 5,463,492 describe cross-linked SPD films usually cured with heat. However, the length of time necessary to cure such a film with heat, i.e., often about 1 hour, is inconveniently long for industrial film-coating processes. By comparison, UV curing of coated films is typically achieved industrially in several seconds, thus enabling a web carrying coated film to move at high speed. Also, in the case of the aforesaid two patents, in order to achieve a relatively uniform distribution of droplets in the cured matrix polymer, it has generally been necessary either to include a separate emulsifier or to incorporate pendant ester groups on the matrix polymer to serve as an emulsifier, i.e., a so-called xe2x80x9ccross-linkable emulsifierxe2x80x9d. Such emulsifiers are required in prior art films due to the inherent instability of the emulsion arising from the complete immiscibility of the matrix polymer and the droplets comprising the liquid light valve suspension. Failure to include an emulsifier in such a system would otherwise permit the droplets to grow too large and thus would result in the formation of an unstable emulsion that could cause problems during film production. Although effective, the cross-linkable emulsifier is difficult to synthesize and does not have a satisfactory, i.e., a sufficiently long, shelf life.
Ultraviolet or electron beam curing of films are well-established industrial techniques. The first attempt to use ultraviolet radiation to cure an SPD film (see U.S. Pat. No. 5,463,491, Example 13) produced a suspension encapsulated within the matrix, but the film was not commercially viable because the liquid suspension in the droplets (or capsules) had been severely degraded, as evidenced by a color change from blue to red. Moreover, the time needed to cure the film with UV radiation, i.e., 10 minutes, was still far too long to be commercially useful. In addition, a mismatch in the indices of refraction of the matrix and suspension in that instance caused unwanted haze, thus negatively affecting the appearance of such SPD films. These problems have been addressed, although not entirely resolved, in subsequent applications filed in the name of the assignee of the present invention (see U.S. Pat. No. 6,114,405) in which the inventors therein have attempted to reduce the cure time from several minutes to several seconds and to match the refractive indices of the polymer system. These improvements resulted in a film that did not exhibit color change, had reduced haze and was commercially more attractive. In the aforesaid U.S. Pat. No. 6,114,405, matching the refractive index values more closely for the suspending medium and the matrix at 1.448 was found to offer a significant improvement in haze reduction. In a subsequent case, U.S. Pat. Ser. No. 09/517,378, filed Mar. 2, 2000, assigned to the Assignee of the present invention, the refractive index of the materials was matched approximately at a value of 1.453, which provided an improvement in emulsion stability without an added emulsifier. Even the emulsions used to form these prior art films, however, are still subject to a degree of instability, which it would be beneficial to eliminate, and which, as described below, is substantially reduced, if not eliminated entirely, by the present invention.
Still further, some prior art SPD films require use of at least one difficult-to-synthesize film-forming monomer. An example of such a monomer is 1,4-bis (hydroxydimethylsilyl) benzene, referred to in Example 24 of U.S. Pat. No. 5,463,492. This monomer is difficult to make and is also very expensive.
Ultraviolet-curable emulsions are described in U.S. Pat. No. 6,114,405 (the ""405 patent) which is assigned to the owner of the present invention. Although such emulsions can be rapidly cured without substantial color change, the resulting films still exhibit certain undesired deficiencies. For example, formation of the film described in the ""405 patent involved the difficult synthesis of the 1,4-bis (hydroxydimethylsilyl) benzene monomer described above (see Example 1). In addition, the liquid suspensions used in forming such films incorporated a high percentage of fluorinated monomers, which are relatively costly and thus add significantly to the expense of the resultant light valve. Also, it was still necessary to use either a cross-linkable emulsifier or a separate emulsifier in order to effect good droplet distribution in the matrix.
Furthermore, the viscosity of the UV-cross-linkable siloxane copolymers made by the method of the ""405 patent was generally very low. For example, in Example 1 of the patent, a method of preparing such a copolymer having a viscosity of only 423 centipoises at 22.9xc2x0 C. is set forth. In order to achieve satisfactory shelf life for such a UV-cross-linkable polymer, end-capping is required. However, some prior art polymeric syntheses, which used Brxc3x6nsted Acid catalysts, caused gels to form, and required that end-capping be done at room temperature, rather than at high temperature, in order to keep the molecular weight up, but the catalyst itself limited the peak molecular weight to less than about 10,000. Yields of only 55-65% were typical.
Another deficiency of the prior art films was their relatively slow decay time (20 secs. and more). While this decay time is acceptable for certain applications, a reduced (i.e., faster) decay time is preferred for many other applications.
U.S. application Ser. No. 09/577,803 filed May 24, 2000 and assigned to the assignee of the present application, eliminates the need for a separate emulsifier or a cross-linkable emulsifier. The current application addresses the other deficiencies of the aforesaid prior art SPD films. In addition, it also includes a procedure for making a matrix polymer with much reduced synthesis time, resulting in reduced cost of the film.
The present invention is directed to improved, radiation cured SPD films, and to light valves incorporating the same, which are produced more simply and quickly, and at a reduced cost, than the films and corresponding light valves previously known in the prior art, while also providing enhanced levels of performance in comparison thereto.
The stability of the emulsion has been substantially enhanced by adjusting the composition of the matrix polymer and the liquid suspending medium such that these materials have a refractive index (RI) within the range of 1.455 to 1.463, more preferably within the range of 1.455 to 1.459, and, most preferably, a refractive index of 1.458. This adjustment, which would be readily understood by one of ordinary skill in this art, requires a reduction/elimination of fluorinated monomer in the suspending polymer and an increase in the amount of phenyl groups on the matrix polymer. These compositional changes, while maintaining immiscibility, increase the affinity between the matrix and liquid suspending medium. This allows small droplets of the liquid suspending medium to exist for substantially longer periods of time without coalescence.
It has additionally been observed that significant reductions in the amount of haze are achieved by maintaining the refractive index of the droplets at a value as close as possible to that of the refractive index of the matrix. Optimally, the best results are achieved when the refractive index values of these components are within about 0.005 of each other. Smaller differences can produce even better results.
The use of the range of refractive indices described above enables addition of low viscosity non-polymeric liquids (fluorinated and non-fluorinated) to the liquid suspending medium. This lowers the overall viscosity of the liquid suspending medium which, in turn, permits rapid orientation and disorientation of the particles in the liquid suspending medium for faster activation (rise) and decay times. Refractive indices lower than 1.458 require more fluorinated non-polymeric liquid, which is a non-solvent for the nitrocellulose coating of the particles. Similarly, refractive indices higher than 1.463 result in enhanced compatibility of the matrix with the liquid suspending medium, especially at elevated temperatures. The matrix polymer is not a good solvent for the nitrocellulose coating of the particles. Non-polymeric liquids useful in the present invention should have a boiling point at atmospheric pressure preferably of at least about 150xc2x0 C., and more preferably over 200xc2x0 C. and a refractive index within the above-described range of 1.455 to 1.463. Moreover, these liquids also should be sufficiently polar as to be immiscible in the matrix material. Some representative examples of these materials include, but are not limited to, dimethyl perfluorosuberate (DMFS) and triethyl trimellitate (TETM).
One may begin with a medium molecular weight suspending polymer of RI 1.463 (which does not comprise any fluorinated monomer) in the liquid suspending medium. It then becomes necessary, however, to incorporate an appropriate combination of fluorinated and non-fluorinated non-polymeric liquids to bring the RI of the liquid suspending medium to 1.458 so as to match that of the matrix polymer. Similar results, i.e., faster activation times and better emulsion stability, have been observed with a matrix resin of RI 1.463 while using a liquid suspending medium of 1.463 RI.
When using a liquid suspending medium having a refractive index within the above-noted ranges, containing non-polymeric liquids, high molecular weight suspending polymer is not utilized. The present invention incorporates, in place of the relatively high molecular weight polymers typically chosen for forming prior art light valves, a lower viscosity (relatively speaking) medium molecular weight polymer system. Methods of making such medium molecular weight polymers are well known among those of ordinary skill in this art. They may include, but are not limited to, those described in U.S. Pat. Nos. 5,463,491, 5,463,492, 5,467,217, 5,728,251 and 6,114,405, which are all assigned to the owner of the present invention.
It has been found that by eliminating the use of high molecular weight polymers and limiting the liquid suspending medium to only such medium molecular weight polymers, a relatively low viscosity droplet is produced for incorporation within the film with no diminution in stability, which, furthermore, has a faster response time as discussed further below. This finding is clearly in contrast to the prior art teaching (discussed above) to the effect that higher viscosity solutions are called for in SPD film applications to ensure the stability of the emulsion used to form the film. As used herein, the term xe2x80x9chigh molecular weightxe2x80x9d is defined as including materials having a viscosity of greater than 5,000 cps when measured at 25xc2x0 C., having number average molecular weights (Mn) greater than 5,000. Alternately, as also used herein a xe2x80x9cmedium molecular weightxe2x80x9d material is one having a viscosity of between 500-5,000 cps when measured at 25xc2x0 C. and a number average molecular weight of from about 1,000 to 5,000. Non-polymeric liquids used in the droplets, however, generally have molecular weights and viscosities below these levels.
Not only does the present invention provide a significant cost reduction by eliminating or reducing the use of relatively expensive fluorinated monomers, it also substantially reduces the time required for synthesizing the matrix polymer. That is, whereas it previously took from about 5 to 8 hours to prepare the cross-linkable emulsifier used in forming prior art films, the inventors have now found that the corresponding operations with the materials of the present invention now require only from one to one and one-half hours, i.e., a savings of from about 4 to 6.5 hours. This is attributable mainly to the optimization of the catalyst quantity and efficient removal of byproduct (water) from the reaction mixture. As is well understood, moreover, by those who work in this field, such a significant time savings further translates to a substantial savings in the costs associated with making the SPD light valves and films of the invention, and thus the resultant products may be offered in a significantly improved form (as described below) at a substantially lower price than prior art products.
Turning next to the improvements in decay time offered by the present invention, it is noted that the decay time of an SPD film or light valve is related to the film or valve""s response time such that the xe2x80x9cdecay timexe2x80x9d as that term is used herein, is defined as the response time required to cover the middle 80% of the transmittance range between the on and off states, once the electrical voltage is removed. Whereas prior art films and associated light valves typically have decay times of twenty seconds or more, the present invention provides films and light valves with a decay time of less than five seconds, i.e., an improvement of over 300%.
The films and corresponding light valves produced according to the present invention thus offer significant savings in the cost and time required for their production, wherein the materials used in forming these products are carefully chosen with matched refractive indices as described above, to provide improved films and light valves having significantly decreased haze coupled with a significant improvement in response time (i.e., as evidenced by the substantial decrease noted in decay time).
The invention is further described below with regard to several preferred embodiments. The invention should not be construed as being limited only to the described embodiments, however, as the following examples are provided as illustrations of, not as limitations to, the invention defined by the appended claims.